#include<stdio.h>
#include<math.h>
double f(double a,double b,double c){
double e=(a+b+c)*0.5;
return e;
}
int main()
{
    double a,b,c=0;
    scanf("%lf%lf%lf",&a,&b,&c);
    double p=f(a,b,c);
    double n=sqrt(p*(p-a)*(p-b)*(p-c));
    printf("%.2lf",n);
    return 0;
}
